1. Field of the Invention
This invention pertains to the field of ballast circuits for powering arc discharge lamps, and is more particularly directed to a fault protection circuit for turning off ballast output power to the lamp load in the event of lamp malfunction.
2. State of the Prior Art
Arc discharge lamps such as fluorescent lamps require relatively high operating voltages, particularly when first initiating the arc discharge in the lamp. Once started, the lamp current must be externally limited to a normal level because arc lamp impedance characteristically drops after start-up. Many so called ballast circuits have been devised for supplying the necessary voltages and currents for powering such lamps.
Arc discharge lamps, for example fluorescent lamp tubes, are subject to certain types of malfunction characteristic of this type of lamp. If the lamp tube is loosened from its electrical socket the lamp connectors or pins may become sufficiently exposed to sustain an arc discharge between the exposed metal parts. Such an arc discharge is undesirable not only because it can severely stress the electrical components of the ballast circuit, but may also pose a fire and electrical shock hazard. Under such circumstances, it is, of course, desirable to shut down electrical power to the lamp immediately. Another type of lamp malfunction occurs with aging of the discharge lamp, manifested by partial rectification of the A.C. current delivered by the ballast to the lamp load. In this case too it is desirable to interrupt electrical power to the lamp load in order to call the attention of maintenance personnel to the need for replacing the lamp. Ballast output protection is particularly needed in arc discharge lamp fixtures installed in aircraft cabin lighting. Aircraft vibration may shake individual lamps loose from their sockets exposing the metal contacts and resulting in arcing. Electrical arcs are clearly undesirable in an environment exposed to combustible fumes from jet fuel.
Various schemes have been devised for sensing abnormal operating conditions of the lamp load in arc discharge fixtures and shutting down the output of the ballast in such event. For example, it is known that certain failure modes, particularly arcing, result in abnormally high transient voltages and currents through the lamp load. This knowledge has been exploited in the past to design protection circuits responsive to such higher than normal voltages or currents. In one known type of protection circuit a voltage comparator has one input connected for sensing the lamp voltage and the other input referenced to a fixed, preset voltage threshold. The output of the comparator is connected for switching off the ballast output in the event that the lamp voltage exceeds the treshhold voltage. This type of output protection circuit works well in ballasts intended for use with a known lamp load, i.e. a given lamp wattage. If the lamp wattage is known in advance, then the normal operating lamp voltage can be anticipated and a reliable reference voltage can be preset for the comparator circuit.
The conventional idea of sampling the output voltage or current of the lamp load and comparing that information against a fixed reference for sensing an over voltage or over current condition is sound. However, the use of a fixed reference will not work when the output lamp load changes, or the ballast is designed for bright and dim lamp operation. This shortcoming is particularly evident with so called universal ballasts designed to supply a range of lamp loads, allowing an end user to install lamps of different wattage as needed. Universal ballasts simplify inventories of electrical supply vendors and facilitate ballast replacement in installed light fixtures, and have come into widespread usage.
What is needed is an output protection circuit which can sense abnormal or unsafe operating conditions at the output of an arc discharge lamp ballast intended for operation with variable lamp loads. Such a protection circuit should be reliable, relatively simple and economical.